Of course, the next step should be to re-enable the FK_CONS
constraint. If you can "guarantee" that the truncate
was done in a "quiet" time (for example, the database
is in restricted mode), then the constraint can be re-enabled in
NOVALIDATE mode. If there is any chance that some illegal
data could have made its way into the A_CHILD table, it makes
sense to follow this with a VALIDATE.

Note that locking the parent table does not lower the risk of
illegal data to be entered, since every DDL you issue (the alter,
the truncate, etc) will immediately commit and thus release any
lock that you had.